A recent observation from our laboratory has suggested that streptococcal M protein has a close structural and physicochemical similarity with mammalian muscle tropomyosin. This relationship is being examined further to determine if the structural relationship with tropomyosin plays a role in the antiphagocytic property of the M protein molecule. Also, the immunological cross-reactions observed between streptococcal components and human heart tissue are being investigated in light of this structural relationship.